Composition of SI/C electro active material

ABSTRACT

A composition comprising a first particulate electroactive material, a particulate graphite material and a binder, wherein at least 50% of the total volume of each said particulate materials is made up of particles having a particle size D50 and wherein a ratio of electroactive material D50 particle size:graphite D50 particle size is up to 4.5:1.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a National Stage Entry of InternationalPatent Application No. PCT/GB2013/050190, filed on Jan. 29, 2013, whichclaims priority to GB patent application No. 1201540.0, filed on Jan.30, 2012, which also claims priority to GB patent application No.1201541.8, filed on Jan. 30, 2012, the entire contents of each of whichare hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to compositions comprising particles of anelectroactive material and additives, and use of said compositions indevices including fuel cells and rechargeable metal ion batteries.

BACKGROUND OF THE INVENTION

Rechargeable metal-ion batteries, for example lithium ion batteries, areextensively used in portable electronic devices such as mobiletelephones and laptops, and are finding increasing application inelectric or hybrid electric vehicles.

Rechargeable metal ion batteries have an anode layer; a cathode layercapable of releasing and re-inserting metal ions; and an electrolytebetween the anode and cathode layers. When the battery cell is fullycharged, metal ions have been transported from the metal-ion-containingcathode layer via the electrolyte into the anode layer. In the case of agraphite-based anode layer of a lithium ion battery, the lithium reactswith the graphite to create the compound Li_(x)C₆ (0<=x<=1). Thegraphite, being the electrochemically active material in the compositeanode layer, has a maximum capacity of 372 mAh/g.

The use of a silicon-based active anode material, which may have ahigher capacity than graphite, is also known.

WO2009/010758 discloses the etching of silicon powder in order to makesilicon material for use in lithium ion batteries.

Xiao et al, J. Electrochem. Soc., Volume 157, Issue 10, pp. A1047-A1051(2010), “Stabilization of Silicon Anode for Li-ion Batteries” disclosesan anode comprising silicon particles and Ketjenblack carbon.

Lestriez et al, Electrochemical and Solid-State Letters, Vol. 12, Issue4, pp. A76-A80 (2009) “Hierarchical and Resilient Conductive Network ofBridged Carbon Nanotubes and Nanofibers for High-Energy Si NegativeElectrodes” discloses a composite electrode containing multiwall carbonnanotubes and vapour-grown nanofibres.

US 2011/163274 discloses an electrode composite of silicon, a carbonnanotube and a carbon nanofibre.

It is an object of the invention to provide an anode composition for ametal ion battery that is capable of maintaining a high capacity.

It is a further objection of the invention to provide a composition forforming an anode of a metal ion battery from a slurry.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a composition comprising afirst particulate electroactive material, a particulate graphitematerial and a binder, wherein at least 50% of the total volume of eachsaid particulate materials is made up of particles having a particlesize D₅₀ and wherein a ratio of electroactive material D₅₀ particlesize:graphite D₅₀ particle size is up to 4.5:1.

It will be understood that the first particulate electroactive materialis different from the particulate graphite material.

Optionally, the ratio is at least 2:1.

Optionally, the ratio is in the range of at least 0.5:1, optionally atleast 0.7:1, optionally at least 2:1-4:1, optionally 3:1-4:1.

Optionally, the particulate graphite material forms 0.5-6 wt % of thecomposition and the ratio is at least 2:1.

Optionally, the particulate graphite material has a BET of at least 3m²/g.

Optionally, the first particulate electroactive material is asilicon-comprising material.

Optionally, the first particulate electroactive material comprisesparticles having a particle core and electroactive pillars extendingfrom the particle core.

Optionally, the pillars of the silicon-comprising particles are siliconpillars.

Optionally, the core of the silicon-comprising particles comprisessilicon.

Optionally, the silicon-comprising particles consist essentially of n-or p-doped silicon and wherein the pillars are integral with the core.

Optionally, the first particulate electroactive material is provided inan amount of at least 50 wt % of the composition.

Optionally, the composition comprises at least one elongatenanostructure material.

Optionally, the first elongate nanostructure has a mean average diameterof at least 100 nm.

Optionally, the composition comprises at least two elongatenanostructure materials.

Optionally, a second elongate carbon nanostructure material has a meanaverage diameter of no more than 90 nm, optionally a mean averagediameter in the range of 40-90 nm.

Optionally, the first elongate nanostructure:second elongatenanostructure weight ratio is in the range 2.5:1 to 20:1.

Optionally, each of the at least one elongate nanostructure materialshas an aspect ratio of at least 50.

Optionally, the first and second carbon elongate nanostructure materialsare each independently selected from carbon nanotubes and carbonnanofibres.

Optionally, the first carbon elongate nanostructure material is ananofibre and the second elongate carbon nanostructure material is ananotube.

Optionally, the at least one elongate carbon nanostructure materials areprovided in a total amount in the range of 0.1-15 weight % of thecomposition.

Optionally, one or more of the elongate carbon nanostructure materialshas a functionalised surface, optionally a surface functionalised with anitrogen-containing group or an oxygen containing group.

Optionally, the graphite is provide in the composition in an amount of1-30 wt %, optionally 1-20 wt %.

Optionally, the crystallite length Lc of the graphite is optionally atleast 50 nm, optionally at least 100 nm.

Optionally, the composition further comprises carbon black.

Optionally, the carbon black is provided in an amount of at least 0.5weight % of the composition, and optionally less than 10 wt % of thecomposition, optionally less than 4 wt % of the composition.

In a second aspect, the invention provides a metal-ion batterycomprising an anode, a cathode and an electrolyte between the anode andthe cathode wherein the anode comprises a composition according to thefirst aspect.

In a third aspect the invention provides a slurry comprising acomposition according to the first aspect and at least one solvent.

In a fourth aspect the invention provides a method of forming ametal-ion battery according to the second aspect, the method comprisingthe step of forming an anode by depositing a slurry according to thethird aspect onto a conductive material and evaporating the at least onesolvent.

Weight percentages of components of a composition described herein arethe weight percentages of those components in a porous or non-poroussolid composition containing all components of the composition. In thecase of a slurry containing a composition, it will be understood thatthe weight of the one or more solvents of the slurry does not form partof the composition weight as described herein.

DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to thedrawings, in which:

FIG. 1 illustrates schematically a metal ion battery according to anembodiment of the invention;

FIG. 2 illustrates schematically a composite electrode according to anembodiment of the invention;

FIG. 3A illustrates schematically a process of forming a pillaredparticle by an etching process;

FIG. 3B illustrates schematically a process of forming a pillaredparticle by growing pillars on a core;

FIG. 4A is a scanning electron microscope image of a compositionaccording to an embodiment of the invention;

FIG. 4B is a magnification of a region of the image of FIG. 4A;

FIG. 4C is a magnification of a region of the image of FIG. 4B;

FIG. 5A illustrates variation of electrode capacity density with cyclenumber for cells according to embodiments of the invention;

FIG. 5B illustrates variation of end charge voltage with cycle numberfor the cells of FIG. 5A;

FIG. 6A illustrates variation of electrode capacity density with cyclenumber for cells according to embodiments of the invention; and

FIG. 6B illustrates variation of end charge voltage with cycle numberfor the cells of FIG. 6A.

FIG. 7 illustrates variation of specific discharge capacity as afunction of the product of the cycle number and electrode capacitydensity in mAh/cm⁻² for cells according to embodiments of the inventionand comparative devices;

FIG. 8 illustrates capacity retention vs. cycle number for exemplarydevices in which dimensions of the graphite additive is varied;

FIG. 9A is a SEM images of graphite CPreme G5; and

FIG. 9B is a SEM images of graphite SFG10.

DETAILED DESCRIPTION OF THE INVENTION

The structure of a rechargeable metal ion battery cell is shown in FIG.1, which is not drawn to any scale. The battery cell includes a singlecell but may also include more than one cell. The battery is preferablya lithium ion battery, but may be a battery of another metal ion, forexample sodium ion and magnesium ion.

The battery cell comprises a current collector for the anode 10, forexample copper, and a current collector for the cathode 12, for examplealuminium, which are both externally connectable to a load or to arecharging source as appropriate. A composite anode layer containingactive silicon particles 14 overlays the current collector 10 and alithium containing metal oxide-based composite cathode layer 16 overlaysthe current collector 12 (for the avoidance of any doubt, the terms“anode” and “cathode” as used herein are used in the sense that thebattery is placed across a load—in this sense the negative electrode isreferred to as the anode and the positive electrode is referred to asthe cathode. “Active material” or “electroactive material” as usedherein means a material which is able to insert into its structure, andrelease therefrom, metal ions such as lithium, sodium, potassium,calcium or magnesium during the respective charging phase anddischarging phase of a battery. Preferably the material is able toinsert and release lithium. Preferred active materials include materialshaving silicon surface at a surface thereof, for example siliconparticles or a composite of a material having a non-silicon core and asurface that is partly or wholly a silicon surface.)

The cathode 12 comprises a material capable of releasing and reabsorbinglithium ions for example a lithium-based metal oxide or phosphate,LiCoO₂, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, LiMn_(x)Ni_(x)Co_(1-2x)O₂ orLiFePO₄.

A liquid electrolyte may be provided between the anode and the cathode.In the example of FIG. 1, a porous plastic spacer or separator 20 isprovided between the anode layer 14 and the lithium containing cathodelayer 16, and a liquid electrolyte material is dispersed within theporous plastic spacer or separator 20, the composite anode layer 14 andthe composite cathode layer 16. The porous plastic spacer or separator20 may be replaced by a polymer electrolyte material and in such casesthe polymer electrolyte material is present within both the compositeanode layer 14 and the composite cathode layer 16. The polymerelectrolyte material can be a solid polymer electrolyte or a gel-typepolymer electrolyte.

When the battery cell is fully charged, lithium has been transportedfrom the lithium containing metal oxide cathode layer 16 via theelectrolyte into the anode layer 14.

A composition according to an embodiment of the invention comprisessilicon-comprising particles, a binder and one or more additives. Eachadditive is preferably a conductive material. Each additive may or maynot be an active material.

The silicon-comprising particles may be structured particles. One formof structured particles are particles having a core, which may or maynot comprise silicon, with silicon-comprising pillars extending from thecore. Another form of structured particles is porous silicon, inparticular macroporous silicon, as described in more detail below.

Additives may be selected from: a first elongate carbon nanostructure;one or more further elongate carbon nanostructures; carbon blackparticles including acetylene black and ketjen black particles; and amaterial containing graphite or graphene particles. Each elongate carbonnanostructure is preferably selected from a nanotube and a nanofibre. A“nanostructure” material as used herein may mean a material comprisingparticles having at least one dimension less than 1 micron, preferablyless than 500 nm, more preferably less than 200 nm.

With reference to FIG. 2, which is not drawn to any scale, a compositionaccording to an embodiment of the invention comprises silicon-comprisingparticles 201, a first elongate nanostructure 203, a second elongatenanostructure 205, carbon black particles 207, graphite particles 209and binder 211. The silicon-comprising particles 201 illustrated in FIG.2 are pillared particles having a core with pillars extending from thecore, however the silicon-comprising particles may or may not carrypillars.

The second elongate nanostructure material may become entangled with thepillars of the pillared silicon particles, and each nanostructure maywrap around some or all of the perimeter of one or more of the pillaredsilicon particle cores, and so may extend electronic conductivity beyondthe pillared particle surface and/or lower barrier to conduction betweenthe pillared particle surface and other conductive species, includingthe binder and other additives of the anode. The second elongatenanostructure may also be entangled with other components of thecomposition, for example graphite (if present).

The pillars, or other structural elements, of the silicon-comprisingparticles 201 may provide anchors for the nanofibres or nanotubes of thesecond elongate nanostructure material 205.

The larger diameter of the first elongate nanostructure material 203 maymake it more rigid than the second elongate nanostructure material 205.The first elongate nanostructure material 203 may provide conductionpaths within the composition that extend along the length of eachnanostructure. These conduction paths may form the framework or supportfor conductive bridges between silicon-comprising particles 201 andbetween the silicon-comprising particles 201 and other components in thecomposite such as graphite particles 209.

Compositions of the invention may include only two different elongatenanostructure materials, for example as illustrated in FIG. 2, or mayinclude three or more different elongate nanostructure materials.

Silicon-Comprising Particles

The silicon-comprising particles may be structured particles. Structuredparticles include particles having a core and pillars extending from thecore, and particles having pores on the particle surface or poresthroughout the particle volume. A surface of a macroporous particle mayhave a substantially continuous network of the particle material at asurface of the particle with spaces, voids or channels within thematerial that may have dimensions of at least 50 nm. Such voids may bepresent throughout the particle volume or may be restricted to regionsof the particle. A particle may have regions of pillars and regions ofpores. The pillars themselves may be microporous or mesoporous.

The silicon-comprising particles in compositions of the invention mayconsist essentially of n- or p-doped silicon or may contain one or morefurther materials. For example, in the case of pillared particles theparticle may be selected from one of the following:

-   -   a particle having a silicon core with pillars extending from and        integral with the silicon core    -   a particle having a non-silicon core of a conductive material,        for example a graphite core, with pillars extending from the        core; and    -   a particle having a non-silicon core of a conductive material,        for example a graphite core, coated with a silicon shell and        having silicon pillars extending from and integral with the        silicon shell.

The pillars may be core-shell structures, the inner core being of adifferent material to the outer shell material and where the core and/orshell contains silicon. In the case where the core and pillars are ofdifferent materials, the core may or may not be an electroactivematerial.

FIG. 3A illustrates a first method of forming pillared particles whereina starting material is etched to form a pillared particle wherein astarting material 301 is exposed to an etching formulation for selectiveetching at the surface of the starting material to produce a pillaredparticle 303 having a core 305 and pillars 307.

It will be appreciated that the volume of the particle core of thepillared particle formed by this method is smaller than the volume ofthe starting material, and the surface of the core is integral with thepillars. The size of the pillared particle may be the same as or lessthan the size of the starting material.

A suitable process for etching a material having silicon at its surfaceis metal-assisted chemical etching (alternatively called galvanicexchange etching or galvanic etching) which comprises treatment of thestarting material with hydrogen fluoride, a source of metal ions, forexample silver or copper, which electrolessly deposit onto the surfaceof the silicon and an oxidant, for example a source of nitrate ions.More detail on suitable etching processes can be found in, for example,Huang et al., Adv. Mater. 23, pp 285-308 (2011).

The etching process may comprise two steps, including a step in whichmetal is formed on the silicon surface of the starting material and anetching step. The presence of an ion that may be reduced is required forthe etching step. Exemplary cations suitable for this purpose includenitrates of silver, iron (III), alkali metals and ammonium. Theformation of pillars is thought to be as a result of etching selectivelytaking place in the areas underlying the electrolessly deposited metal.

The metal deposition and etching steps may take place in a singlesolution or may take place in two separate solutions.

Metal used in the etching process may be recovered from the reactionmixture for re-use, particularly if it is an expensive metal such assilver.

Exemplary etching processes suitable for forming pillared particles aredisclosed in WO 2009/010758 and in WO 2010/040985.

Other etching processes that may be employed include reactive ionetching, and other chemical or electrochemical etching techniques,optionally using lithography to define the pillar array.

If the pillared particle comprises a first material at its core centrewith a shell formed from a second material, for example carbon coatedwith silicon, then this particle may be formed by etching ofsilicon-coated carbon to a depth of less than the thickness of thesilicon shell in order to form a pillared particle with a compositecarbon/silicon core.

Etching may be to a depth of less than 2-10 microns, optionally at least0.5 microns, to form pillars having a height of up to 10 microns. Thepillars may have any shape. For example, the pillars may be branched orunbranched; substantially straight or bent; and of a substantiallyconstant thickness or tapering.

The pillars may be formed on or attached to a particle core usingmethods such as growing, adhering or fusing pillars onto a core orgrowing pillars out of a core. FIG. 3B illustrates a second method offorming pillared particles wherein pillars 307, preferably siliconpillars, for example silicon nanowires, are grown on or attached to astarting material 301 such as a silicon or carbon (e.g. graphite orgraphene) starting material. The volume of the particle core 305 of theresultant pillared particle 303 may be substantially the same as thevolume of the starting material 301. In other words, the surface of thestarting material may provide the surface of the particle core 305 fromwhich the pillars 307 extend.

Exemplary methods for growing pillars include chemical vapour deposition(CVD) and fluidised bed reactors utilising the vapour-liquid-solid (VLS)method. The VLS method comprises the steps of forming a liquid alloydroplet on the starting material surface where a wire is to be grownfollowed by introduction in vapour form of the substance to form apillar, which diffuses into the liquid. Supersaturation and nucleationat the liquid/solid interface leads to axial crystal growth. Thecatalyst material used to form the liquid alloy droplet may for exampleinclude Au, Ni or Sn.

Nanowires may be grown on one or more surfaces of a starting material.

Pillars may also be produced on the surface of the starting materialusing thermal plasma or laser ablation techniques.

The pillars may also be formed by nanowire growth out of the startingmaterial using methods such as a solid-liquid-solid growth technique. Inone example silicon or silicon-based starting material granules arecoated with catalyst particles (e.g. Ni) and heated so that a liquidalloy droplet forms on the surface whilst a vapour is introducedcontaining another element. The vapour induces condensation of a productcontaining the starting material and the other element from the vapour,producing growth of a nanowire out of the starting material. The processis stopped before all of the starting material is subsumed intonanowires to produce a pillared particle. In this method the core of thepillared particle will be smaller than the starting material.

Silicon pillars grown on or out of starting materials may be grown asundoped silicon or they may be doped by introducing a dopant during thenanowire growth or during a post-growth processing step.

The pillars are spaced apart on the surface of the core. In onearrangement, substantially all pillars may be spaced apart. In anotherarrangement, some of the pillars may be clustered together.

The starting material for the particle core is preferably in particulateform, for example a powder, and the particles of the starting materialmay have any shape. For example, the starting material particles may becuboid, cuboidal, substantially spherical or spheroid or flake-like inshape. The particle surfaces may be smooth, rough or angular and theparticles may be multi-faceted or have a single continuously curvedsurface. The particles may be porous or non-porous.

A cuboid, multifaceted, flake-like, substantially spherical or spheroidstarting material may be obtained by grinding a precursor material, forexample doped or undoped silicon as described below, and then sieving orclassifying the ground precursor material. Exemplary grinding methodsinclude power grinding, jet milling or ball milling. Depending on thesize, shape and form of the precursor material, different millingprocesses can produce particles of different size, shape and surfacesmoothness. Flake-like particles may also be made by breakingup/grinding flat sheets of the precursor material. The startingmaterials may alternatively be made by various deposition, thermalplasma or laser ablation techniques by depositing a film or particulatelayer onto a substrate and by removing the film or particulate layerfrom the substrate and grinding it into smaller particles as necessary.

The starting material may comprise particles of substantially the samesize. Alternatively, the starting material may have a distribution ofparticle sizes. In either case, sieves and/or classifiers may be used toremove some or all starting materials having maximum or minimum sizesoutside desired size limits.

In the case where pillared particles are formed by etching a materialcomprising silicon, the starting material may be undoped silicon ordoped silicon of either the p- or n-type or a mixture, such as silicondoped with germanium, phosphorous, aluminium, silver, boron and/or zinc.It is preferred that the silicon has some doping since it improves theconductivity of the silicon during the etching process as compared toundoped silicon. The starting material is optionally p-doped siliconhaving 10¹⁹ to 10²⁰ carriers/cc.

Silicon granules used to form the pillared particles may have asilicon-purity of 90.00% or over by mass, for example 95.0% to 99.99%,optionally 98% to 99.98%.

The starting material may be relatively high purity silicon wafers usedin the semiconductor industry formed into granules. Alternatively, thegranules may be relatively low purity metallurgical grade silicon, whichis available commercially and which may have a silicon purity of atleast 98%; metallurgical grade silicon is particularly suitable becauseof the relatively low cost and the relatively high density of defects(compared to silicon wafers used in the semiconductor industry). Thisleads to a low resistance and hence high conductivity, which isadvantageous when the pillar particles or fibres are used as anodematerial in rechargeable cells. Impurities present in metallurgicalgrade silicon may include Iron, Aluminium, Nickel, Boron, Calcium,Copper, Titanium, and Vanadium, oxygen, carbon, manganese andphosphorus. Certain impurities such as Al, C, Cu, P and B can furtherimprove the conductivity of the starting material by providing dopingelements. Such silicon may be ground and graded as discussed above. Anexample of such silicon is “Silgrain™” from Elkem of Norway, which canbe ground and sieved (if necessary) to produce silicon granules, thatmay be cuboidal and/or spheroidal.

The granules used for etching may be crystalline, for example mono- orpoly-crystalline with a crystallite size equal to or greater than therequired pillar height. The polycrystalline granules may comprise anynumber of crystals, for example two or more.

Where the pillared particles are made by a growth of silicon pillars asdescribed above, the starting material may comprise an electroactivematerial, and may comprise metal or carbon based particles. Carbon basedstarting materials may comprise soft carbon, hard carbon, natural andsynthetic graphite, graphite oxide, fluorinated graphite,fluorine-intercalated graphite, or graphene.

Graphene based starting materials may comprise particles comprising aplurality of stacked graphene nanosheets (GNS) and/or oxidised graphenenanosheets (ox-GNS), sometimes called Graphite Nano Platelets (GNP) oralternatively nano Graphene Platelets (NGP). NGP (or GNP) may havethicknesses of at least a few nanometers (e.g. at least 2 nm) and largerdimensions of up to 100 μm, preferably less than 40 μm. Materialscomprising a plurality of stacked graphene sheets are graphitematerials. Methods of making graphene based particles includeexfoliation techniques (physical, chemical or mechanical), unzipping ofMWCNT or CNT, epitaxial growth by CVD and the reduction of sugars.

The core of the silicon-comprising particle illustrated in FIG. 3 issubstantially spherical, however the particle core may have any shape,including substantially spherical, spheroidal (oblate and prolate), andirregular or regular multifaceted shapes (including substantially cubeand cuboidal shapes). The particle core surfaces from which the pillarsextend may be smooth, rough or angular and may be multi-faceted or havea single continuously curved surface. The particle core may be porous ornon-porous. A cuboidal core may be in the form of a flake, having athickness that is substantially smaller than its length or width suchthat the core has only two major surfaces.

The aspect ratio of a pillared particle core having dimensions of lengthL, width W and thickness T is a ratio of the length L to thickness T(L:T) or width W to thickness T (W:T) of the core, wherein the thicknessT is taken to be the smallest of the 3 dimensions of the particle core.The aspect ratio is 1:1 in the case of a perfectly spherical core.Prolate or oblate spheroid, cuboidal or irregular shaped corespreferably have an aspect ratio of at least 1.2:1, more preferably atleast 1.5:1 and most preferably at least 2:1. Flake like cores may havean aspect ratio of at least 3:1.

In the case of a substantially spherical core, pillars may be providedon one or both hemispheres of the core. In the case of a multifacetedcore, pillars may be provided on one or more (including all) surfaces ofthe core. For example, in the case of a flake core the pillars may beprovided on only one of the major surfaces of the flake or on both majorsurfaces.

The core material may be selected to be a relatively high conductivitymaterial, for example a material with higher conductivity than thepillars, and at least one surface of the core material may remainuncovered with pillars. The at least one exposed surface of theconductive core material may provide higher conductivity of a compositeanode layer comprising the pillared particles as compared to a particlein which all surfaces are covered with pillars.

The silicon particles may have at least one smallest dimension less thanone micron. Preferably the smallest dimension is less than 500 nm, morepreferably less than 300 nm. The smallest dimension may be more than 0.5nm. The smallest dimension of a particle is defined as the size of thesmallest dimension of an element of the particle such as the diameterfor a rod, fibre or wire, the smallest diameter of a cuboid or spheroidor the smallest average thickness for a ribbon, flake or sheet where theparticle may consist of the rod, fibre, wire, cuboid, spheroid, ribbon,flake or sheet itself or may comprise the rod, fibre, wire, cuboid,spheroid, ribbon, flake or sheet as a structural element of theparticle.

Preferably the particles have a largest dimension that is no more than100 μm, more preferably, no more than 50 μm and especially no more than30 μm.

Particle sizes may be measured using optical methods, for examplescanning electron microscopy.

Preferably at least 20%, more preferably at least 50% of the siliconparticles have smallest dimensions in the ranges defined herein.Particle size distribution may be measured using laser diffractionmethods, for example using a MasterSizer® as described in more detailbelow, or optical digital imaging methods.

Elongate Carbon Nanostructure Materials

A composition of the invention may include one, two or more elongatecarbon nanostructure materials in addition to the particulate graphitematerial described below. A first elongate carbon nanostructure materialmay have a diameter (or smallest dimension) that is larger than that ofthe second elongate carbon nanostructure. The second nanostructurematerial may have a higher surface area per unit mass than the firstnanostructure material. The first elongate nanostructure material mayhave a large enough diameter so that the nanostructure is relativelystraight and rigid whereas the second elongate nanostructure may have asmall enough diameter such that it can be flexible and curved or bentwithin the composite. Preferably the diameter (or smallest dimension) ofthe first elongate carbon nanostructure is at least 100 nm. Preferablythe diameter (or smallest dimension) of the second elongate carbonnanostructure is less than 100 nm, more preferably less than 90 nm, morepreferably less than 80 nm. Preferably, both the average thickness andaverage width of each of the first and second elongate carbonnanostructures is less than 500 nm.

Each of the elongate carbon nanostructure materials may have a largeaspect ratio, the aspect ratio being the ratio of the largest andsmallest dimensions of the material. Preferably, the aspect ratio of thefirst elongate carbon nanostructure is in the range of about 40 to 180.Preferably the aspect ratio of the second carbon nanostructure is in therange of 200 to 500.

Elongate nanostructures may be selected from nanofibres and/or nanotubesand thin ribbons.

Nanotubes may be single-walled or multi-walled. Preferably, carbonnanotubes used in compositions of the invention are multi-walled. Wallsof the nanotubes may be of graphene sheets.

Nanofibres may be solid carbon fibres or may have a narrow hollow core,and may be formed from stacked graphene sheets. An example of a suitablenanofibre material is VGCF® supplied by Showa Denko KK.

Optionally, the elongate nanostructures have a mean average length inthe range of 3-50 μm. Preferably the length of the first elongatenanostructure material is in the range 5-30 μm.

Preferably the surface area of each elongate nanostructure material isno more than 100 m²/g and at least 1 m²/g.

The first elongate nanostructure may be a nanofibre having a surfacearea in the range of 10-20 m²/g

The second elongate nanostructure may be a nanotube have a surface areain the range of 40-80 m²/g.

The carbon nanostructures may be functionalised to improve adhesion orconnection to other components in the composition, especially thesilicon-comprising particles. For example carbon nanotubes can befunctionalised with oxygen-containing groups, for example COOH, OH, COand nitrogen containing groups, for example NH₂. The second elongatenanostructure may be a carbon nanotube functionalised with COOH groupswhich may promote connectivity to the surface of silicon-comprisingparticles or other electroactive particles.

A composition including a binder, silicon-comprising particles, two ormore different elongate carbon nanostructure materials and any furtheradditives may include each of the elongate nanostructure materials in anamount in the range of 0.25-20 weight %, optionally 0.25-10 wt % of thecomposition. The total amount of the two or more different elongatenanostructure materials in the composition may be in the range of 2-25weight percent, optionally 3-13 weight percent.

Carbon Black

The composition may comprise carbon black, which may be characterised asa highly conducting particulate carbon, quasigraphitic in nature,composed of aggregates having a complex configuration (including but notlimited to chain-like agglomerates) and of colloidal dimensions. Carbonblack is typically made via the thermal decomposition and partialcombustion of hydrocarbons. Various types of carbon black are available,including acetylene blacks. Examples of commercial products includeKetjen Black® EC600JD or EC300J supplied by AkzoNobel, Vulcan® XC72Rmanufactured by Cabot Corp, TokaBlack® 5500, 4500, 4400 or 4300manufactured by Tokai Carbon Co., LTD. and DenkaBlack® FX-35 or HS-100manufactured by Denki Kagaku Kogyo Kabushiki Kaisha. The composition maycomprise a single type of carbon black or a blend of one or more typesof carbon black. The carbon black particles may have dimensions in therange of 10-100 nm and a surface area in excess of 50 m²/g.

A composition including a binder, silicon-comprising particles, a firstelongate carbon nanostructure and a second elongate carbonnanostructure, carbon black additive(s) and any further additives mayinclude carbon black (of a single type or a blend of a plurality oftypes) in an amount of at least 0.25 weight % of the composition, andoptionally less than 10 wt % of the composition. Preferably, the carbonblack is present in an amount in the range 0.5 wt % to 4 wt % of thetotal composition. Ketjen Black EC600JD with an average particle size of20-40 nm and a surface area of >1000 m²/g is particularly preferred asan additive.

Graphite Particles

The composition may contain graphite particles, optionally graphiteflakes. Optionally the graphite is synthetic graphite.

The crystallite length Lc of the graphite particles is optionally atleast 50 nm, optionally at least 100 nm. Graphite with a highercrystallite length Lc may be preferable as this may provide higherconductivity, and higher overall conductivity of the composite. Suitablecommercial products of graphite particles may include Timrex® SFG6,SFG10, SFG15, KS4 or KS6 manufactured by Timcal Ltd, 4287 or HPM850manufactured by Asbury.

Graphite present in an anode of a metal ion battery may function as anactive material. Active graphite may provide for a larger number ofcharge/discharge cycles without significant loss of capacity than activesilicon, whereas silicon may provide for a higher capacity thangraphite. Accordingly, an electrode composition having bothsilicon-comprising active particles and a graphite active material mayprovide a metal ion battery, for example lithium ion battery, with theadvantages of both high capacity and a large number of charge/dischargecycles. Depending on the type of graphite material and thecharge/discharge conditions, the graphite additive in a silicon basedcomposition may not be fully lithiated during charging and may have anegligible or zero contribution to the electrode capacity above that ofthe silicon based material. It may be used primarily to improve theoverall conductivity of the composition.

Graphite present in the composition may also improve coating propertiesof a slurry of the composition as compared to a composition in whichgraphite is absent.

Graphite particles may be provided as a powder having a D₅₀ size asmeasured using laser diffractometry of less than 50 microns, optionallyless than 25 microns. Graphite particles may have a BET (Brunauer EmmettTeller) surface area of at least 3 m2/g, optionally at least 5 m²/g or10 m²/g. Optionally, the graphite particles have a BET value of no morethan 300 m²/g, optionally no more than 250 m²/g, optionally no more than100 m²/g, optionally no more than 50 m²/g.

Dn as used herein (for example, D₅₀ or D₉₀) means that at least n % ofthe volume of the material is formed from particles have a measuredspherical equivalent volume diameter equal to or less than theidentified diameter.

Flake-like graphite particles may have a length, height and thicknesswherein both length and width of the particles are each independently onaverage at least 5 times, optionally at least 10 times, the thickness ofthe particles. Average thickness of graphite flakes may be in the rangeof less than 1 micron, optionally 75-300 nm. Average dimensions may bemeasured from an SEM image of a sample of the particles.

A composition including a binder, silicon-comprising particles, graphiteand any further additives may include graphite in an amount in the rangeof at least 0.5 or at least 1 wt % graphite, optionally 2-30 wt %,optionally 2-15 wt %. The present inventors have surprisingly found thatthe performance of a metal-ion battery having a composite anodecontaining both silicon-comprising particles and graphite particles maybe affected by the size ratio of the silicon-comprising particles to thegraphite particles.

Graphite additives as described herein may be graphene-based particlescomprising a plurality of stacked graphene sheets. Graphene-basedparticles may comprise a plurality of stacked graphene nanosheets (GNS)and/or oxidised graphene nanosheets (ox-GNS), sometimes called GraphiteNano Platelets (GNP) or alternatively nano Graphene Platelets (NGP). NGP(or GNP) may have thicknesses of at least a few nanometers (e.g. atleast 2 nm) and larger dimensions of up to 100 μm, preferably less than40 μm. Methods of making graphene-based particles include exfoliationtechniques (physical, chemical or mechanical), unzipping of MWCNT orCNT, epitaxial growth by CVD and the reduction of sugars.

Binder

The binder may be provided to provide cohesion of the particles and, inthe case of use in a metal ion battery, for adhesion of the compositionto an anode current collector.

The binder material may be a polymeric material, for example polyimide,polyacrylic acid (PAA) and alkali metal salts thereof, polyvinylalchol(PVA), polyvinylidene fluoride (PVDF) and sodium carboxymethylcellulose(Na-CMC) or rubber based binders such as SBR. Mixtures of differentbinder materials may be used.

The binder may be provided in an amount in the range of 5-30 wt % of thecomposition.

Composition

The silicon particles and the carbon additives and any other additivesmay each be provided in the form of a powder or slurry for ease ofmixing and blending. For example a slurry can be made by mixing thesilicon particles or carbon additives with an appropriate amount ofaqueous (e.g. water) and/or non-aqueous (e.g. NMP) solvent. A slurry ofa composition comprising the silicon particles, carbon additives and anyother additives may be made by mixing all elements together with asolvent or alternatively may be made by first making more than oneslurry, each slurry comprising one or more the individual elements ofthe composition in a solvent and then combining the separate slurriestogether to create a slurry containing all elements of the composition.The solvents of the separate starting slurries may be the same or may bedifferent, as long as they are miscible when combined. A binder materialwith or without a solvent may also be added and blended to thecomposition or slurry. The resulting slurry may be deposited onto asubstrate and dried to remove the solvent to form a composition for theelectrode of a metal-ion battery.

The inventors have recognised that if a metal ion battery comprising anegative electrode comprising a silicon containing electroactivematerial is to cycle with a high capacity (for example, in excess of 500mAh per gram of active material) for in excess of 100-300charge/discharge cycles, then the electrode composite structure shouldbe uniformly porous and electronically well connected and designed toaccommodate the volume changes of the electroactive material duringcycling without mechanical or electronic disconnection of the activematerial from the composite structure.

In order to achieve this, the components within the composite may havemoderate values of surface area per unit mass. A high surface area mayprovide higher reactivity of the active material or improvedconductivity from the additives, however if the surface area of thecomponents is too high, excessive formation of a solid-electrolyteinterphase (SEI) layer may increase metal ion loss, cause reductioncycle life and cause reduction in porosity. In addition, an excessivesurface area of the additives will require a higher content of binder inthe composition to effectively bind the components of the compositetogether and to adhere it to the current collector—which may reduce theoverall volumetric capacity and make it difficult to provide anappropriate level of porosity in the composition.

When the composition is mixed with a solvent to form a slurry fordepositing the composition onto a current collector, the mix ofcomponents with different shapes and varying volumes is preferably suchthat slurry comprise a uniform mixture with all components equallydispersed and of sufficiently low viscosity to enable thin, uniformcoatings to be prepared.

The inventors have discovered that a negative electrode with acomposition having the following properties may provide improved cyclingperformance as described above:

(a) At least 50 wt % active material and no more than 80 wt %, theactive material preferably comprising structured silicon particles

(b) Binder in the range of 5-30 wt %, preferably 10-20 wt %.

(c) First elongate carbon nanostructure material comprisingnanostructures with a smallest dimension of more than 100 nm in theamount of 0.25 to 20 wt %, preferably 3-7 wt %

(d) Second elongate carbon nanostructure material comprisingnanostructures with a smallest dimension of less than 100 nm, preferablyin the range 30-80 nm, in the amount of 0.25 to 20 wt %, more preferably2-8 wt %.

(e) Carbon black in the range 0.25 to 10 wt %, preferably 0.5 to 4 wt %.

(f) Graphite particles and/or other additives, fillers and spacers inthe range 2-30 wt %

(g) A porosity of at least 10-80%, preferably 20-60%.

wherein the total percentage of the above components adds up to 100 wt%. Preferably the total amount of the first and second elongate carbonnanostructures (c and d) in the composition is in the range 2-25 wt %,especially 3-13 wt %. Preferably the ratio of the mass of the firstelongate carbon nanostructure material to the mass of the secondelongate carbon nanostructure material is no more than 5:1, mostpreferably the ratio is in the range 0.1:1 to 5:1 and especially 0.5:1to 2:1.

Preferably the composition comprises structured silicon particles asdescribed above. The inventors have discovered that all three carboncomponents c, d and e, within the weight amounts described above mayproduce a negative electrode with excellent cyclability. Without wishingto be bound by theory, it is believed that by using elongate carbonnanostructures such as MWCNT with diameters in the range 30-80 nm and inthe amounts described above, the MWCNT can become entangled with thestructural features of the silicon structured particles to form shortrange conductive networks without excessive filling of the voids orspaces between the said structural features that are necessary toprovide space for silicon expansion and access of electrolyte. Thelarger diameter, rigid first elongate carbon nanostructures, such asVGCF, provide conductive bridges for longer range electronic connectionsand help to provide a strong mechanical framework within the compositionto withstand the volume expansion and contraction of the active materialduring cycling. It is believed that the highly dispersed carbon blackmay provide sufficient conductivity in the remaining locations withinthe composition. However if an excessive amount of any of the carbonadditives is used then the effectiveness of the binder may be reducedand the uniformity of the composition may be reduced.

The composition may be formed by mixing the components of thecomposition, a solvent and optionally one or more of a surfactant,dispersant or porogen, and stirring the mixture. Two or more of thecomponents may first be mixed together in a solvent before being addedto the other components for a final mixing stage. The composition maythen be deposited on a substrate and dried so that the solvent isevaporated to form a porous composite film.

EXAMPLES

Materials

Compositions were prepared with components selected from the followingmaterials:

Pillared silicon particles formed by etching starting silicon particlesavailable as “Silgrain™” from Elkem of Norway, wherein the startingsilicon particles have a D₅₀ particle size of 11.5-12.5 microns, or24.5-25.5 microns as measured using a Mastersizer™ particle sizeanalyzer available from Malvern Instruments Ltd. It will be understoodthat the resultant pillared particle may have a D₅₀ that is smaller thanthat of the starting material, for example up to 2 or 4 microns smallerrespectively.

VGCF carbon nanofibres available from Showa Denko, having an averagediameter of 150 nm, an average length of 10-20 microns and a surfacearea of 13 m²/g.

Multiwalled carbon nanotubes from CheapTubes Inc having an averagediameter of 50-80 nm, an average length of 15-20 microns and a surfacearea of 55-75 m²/g (hereinafter “MWCNT”).

Carbon black material available from AzkoNobel as Ketjenblack® EC600-JDhaving a surface area of 1400 m²/g and an average particle size of 20-40nm.

Carbon black material available from Denka as Denka black having asurface area of 69 m²/g and an average particle size of 35 nm.

Graphite available as TIMCAL TIMREX® KS4, KS6, SFG6 and SFG10 havingD₁₀, D₅₀ and D₉₀ values (measured using a MasterSizer particle sizeanalyser) and BET values as given in Table 2.

A sodium polyacrylate binder, hereinafter referred to as “NaPAA” wasformed by partially neutralising commercially available polyacrylicPAA450K using sodium carbonate or sodium hydroxide to a 70% degree ofneutralisation. A distribution of the particle sizes of a powder ofstarting material particles used to form pillared particles may bemeasured by laser diffraction, in which the particles being measured aretypically assumed to be spherical, and in which particle size isexpressed as a spherical equivalent volume diameter, for example usingthe Mastersizer™ particle size analyzer available from MalvernInstruments Ltd. A spherical equivalent volume diameter is the diameterof a sphere with the same volume as that of the particle being measured.If all particles in the powder being measured have the same density thenthe spherical equivalent volume diameter is equal to the sphericalequivalent mass diameter which is the diameter of a sphere that has thesame mass as the mass of the particle being measured. For measurementthe powder is typically dispersed in a medium with a refractive indexthat is different to the refractive index of the powder material. Asuitable dispersant for powders of the present invention is water. For apowder with different size dimensions such a particle size analyserprovides a spherical equivalent volume diameter distribution curve.

FIG. 4A is a SEM image of a composition containing each of theaforementioned components following formation of a slurry of thecomposition and deposition of the composition onto a copper currentcollector and evaporation of the slurry solvent to form an anode layer.

The second elongate nanostructures 205, which in this case aremultiwalled carbon nanotubes, are entangled with the silicon-comprisingparticles 201, which in this case are pillared silicon particles. Thefirst elongate nanostructures 203, in this case a nanofibre, providesconductivity over a relatively long range, as shown for the annotatednanofibre 203 bridging two silicon particles.

The nanotubes provide medium range conductivity. Referring to FIGS. 4Band 4C, it can be seen that nanotubes 205 form a bridge extending acrosstwo silicon particles 201. The nanotubes and nanoparticles also providefor improved conductivity between the silicon particles and graphiteflakes 209 of the composition.

General Device Process 1

Swagelok™-style test cells were constructed using an anode comprising acomposition comprising silicon pillared particles as the active materialdeposited with a coat weight of 13.5-15.5 grams of silicon per m² onto a10 μm thick copper foil, an NCA cathode(Li_(1+x)Ni_(0.8)Co_(0.15)Al_(0.05)O₂) on an aluminium foil and a Tonenseparator between the two electrodes. The electrodes and separator werewetted with an electrolyte solution of 1M LiPF₆ in EC/EMC containing VC(vinylene carbonate, 3 wt %, FEC (fluoroethylene carbonate, 10 wt %) andCO₂ (0.2 wt %) as additives. The capacity of the NCA cathode was 3 timeshigher than the capacity of ppSi in the composite electrode that wasdesigned to operate at 1200 mAh/g. The silicon pillared particles wereprepared using metal-assisted etching of metallurgical grade siliconparticles (with a silicon purity of 99.7-99.95 wt %), to form irregularshaped pillars of lengths 1.5-2.5 μm and thicknesses of 40-150 nm suchthat the average mass of the pillars was 20-40% of the total siliconmass. The cells were cycled in such a way that the ppSi was charged to1200 mAh/g and discharged to a cut-off voltage of 2.5V. The cycling ratewas C/2 for both charge and discharge. The electrode area was 1.13 cm².

Device Example 1

Compositions of the following materials in the following weight ratioswere prepared:

70 wt % pillared silicon particles (pillared particle D₁₀=11 μm, D₅₀=21microns, D₉₀=39 μm)

12 wt % binder NaPAA

6 wt % graphite

12 wt % made up of VGCF:multi-wall carbon nanotubes:EC600:Ketjenblack®EC600-JD:Denka black in the ratio of 4:1:1:2. Graphite was varied asshown in Table 1.

TABLE 1 Graphite Graphite Graphite Silicon:Graphite Graphite BET First,second Graphite D₁₀ D₅₀ D₉₀ D₅₀ surface area and third cycle ExampleType (microns) (microns) (microns) ratio (m²/g) efficiencies (%)Comparative KS4 1.2 2.4 4.7 8.75 26 57, 85, 100 Example 1 ComparativeKS6 1.6 3.4 6.5 6.18 20 63, 87, 100 Example 2 Comparative SFG6 1.7 3.56.5 6.00 17 63, 87, 100 Example 3 Example 1 SFG10 2.8 6.6 12.8 3.18 12.564, 86, 100

The similarities in efficiencies for different sizes of graphiteindicate that graphite size has little or no effect on first andsubsequent cycle efficiencies.

These compositions were used to prepare lithium-ion cells according tothe General Device Process. The devices had first, second and thirdcycle efficiencies as shown in Table 1.

FIG. 5A depicts the evolution of the capacity density of the cells ofComparative Examples 1-3 and Example 1, and FIG. 5B shows the evolutionof the end of charge voltage for these cells with cycle number. The endof charge voltage was limited to 4.3V.

FIG. 5B shows that cell resistance increases fastest for the anode ofExample Comparative Example 3, containing SFG6. In particular, the cellresistance of Comparative Example 3 increases faster than forComparative Example 1, containing KS4.

Example 1 delivers the highest capacity density over 350 cycles.

Device Example 2

Devices were prepared as described with reference to Example 1 exceptthat the pillared silicon particles had a D₅₀ size of 11 μm, a D₉₀ sizeof 20 μm and a D₁₀ size of 6 μm and the graphite was varied as shown inTable 2.

TABLE 2 Graphite First, second BET and third Graphite GraphiteSilicon:Graphite Composition surface cycle D₅₀ D₉₀ D₅₀ coat weight areaefficiencies Example Graphite (microns) (microns) ratio (g-Si/m²) (m²/g)(%) Comparative KS4 2.4 4.7 4.58 13.8 26 69, 80, 100 Example 4Comparative KS4 2.4 4.7 4.58 13.6 26 81, 81, 100 Example 5 Example 2SFG6 3.5 6.5 3.14 14 17 77, 81, 100

The measured cycling efficiencies in Table 2 indicate that theperformance improvement in Example 2 is not simply down to the lowersurface area of SFG6 leading to less SEI layer being formed in the firstfew cycles.

In contrast to the relative performance described above with referenceto Comparative Examples 1 and 3, FIG. 6A shows that Example 2 containingSFG6 with pillared silicon particles having a D₅₀ of 11 micronsmaintains its capacity for a larger number of cycles than ComparativeExamples 4 and 5 containing KS4, and FIG. 6B shows that cell resistanceincreases fastest for the anode of Comparative Examples 4 and 5,indicating a relationship between silicon particle size and graphitesize for optimum performance. Preferably, the silicon:graphite D₅₀ ratiois at least 0:7:1 or at least 2:1, and optionally it is no more than4.5:1, optionally no more than 4:1.

The preferred silicon:graphite D₅₀ ratio may depend on the quantity ofgraphite present in the composite anode. Optionally, thesilicon:graphite D₅₀ ratio

is in the range 2:1 to 4:1, optionally 3:1 to 4:1 at graphite weightpercentages of up to 6 wt %. Optionally, the silicon:graphite D₅₀ ratiois in the range 0.7:1 to 4.5:1 at graphite weight percentages above 6 wt%

Device Examples 3-7

Compositions of the following materials in the following weight ratioswere prepared:

70 wt % pillared silicon particles (pillared particle D₅₀=11.1 microns)

14 wt % binder NaPAA

4 wt % graphite SFG6

12 wt % made up of elongate nanostructures, VGCF and EC600, as shown inTable 3.

TABLE 3 Nanotube Carbon black First, second and MWCNT VGCF EC600 thirdcycle efficiencies Example (wt %) (wt %) (wt %) (%) 3 5 5 2 36, 70, 1004 8 1 3 73, 100, 79 5 0 11 1 79, 76, 100 6 11 0 1 73, 99, 80 7 7 1 4 72,72, 100

These compositions were used to prepare lithium-ion cells according tothe General Device Process. The devices had first, second and thirdcycle efficiencies as shown in Table 3. The nth cycle efficiency is theratio of the discharge capacity to the preceding charge capacity andprovides an indication of the amount of lithium lost or retained withinthe anode or other cell components during the nth charge-dischargecycle, for example due to formation of the SEI layer.

With reference to FIG. 7, normalised capacity starts to decrease at alower cycle number for devices having compositions in which one of VGCFand MWCNT is not present than for Example 8, in which both VGCF andMWCNT are present.

With reference to FIG. 7, decrease in capacity is fastest for Example 7.Without wishing to be bound by any theory, it is believed that the highlevel of carbon black in Example 7 may result in a high level ofabsorption of the binder due to the high surface area per unit mass ofthe carbon black. Preferably, the weight ratio given by the combinedmass of the elongate carbon nanostructures to the mass of the carbonblack particles is in the range 3:1 to 20:1.

General Device Process 2

Swagelok™-style test cells were constructed using an anode comprising acomposition comprising either silicon pillared particles or unetched(non-pillared) silicon particles as the active material deposited with acoat weight of 30 grams of silicon per m²±5% (total coat weight is about44 g/m²) onto a 10 μm thick copper foil, an NCA cathode(Li_(1+x)Ni_(0.8)Co_(0.15)Al_(0.05)O₂) on an aluminium foil and a Tonenseparator between the two electrodes. The electrodes and separator werewetted with a 1M electrolyte solution of 95 wt % LiPF₆ and 5 wt % LiBOBin FEC/EMC (1:1 by volume) containing 3 weight % VC additive. Thecathode:anode capacity ratio was 3.4:3.

The composite anode had a composition of:

-   -   70 wt % silicon (pillared or non-pillared)    -   14 wt % binder of 70% neutralised Na-PAA with a molecular weight        of 450 k    -   12 wt % graphite additive as set out in Table 4    -   4 weight percent of a mixture of VGCF:CNT:CB in the weight ratio        5:5:2

The silicon pillared particles used as the active material were preparedusing metal-assisted etching of metallurgical grade silicon particles(with a silicon purity of 99.7-99.95 wt %) to form pillared particleswherein D50=11.4 microns and BET=12.2 m²/g,

The metallurgical grade silicon powder used as the active material hadD50=4.6 microns and BET=2 m²/g.

The cells were charged to 1000 mAh/g at constant current and constantvoltage. The first cycle was at C/25 between 4.2 to 3V. The subsequentcycles were at C/3 between 4.1 and 3 V.

First cycle losses were measured using different sections of the anodecoatings in half test cells (vs lithium metal foil). This is done so wecan exclude the loss due to the cathode.

TABLE 4 Graphite Type D50 BET (supplier) (μm) (m²/g) SFG10 (Timcal) 6.612 SFG6 (Timcal) 3.5 17 4827 (Asbury) 1.64 249 HPM850 (Asbury) 4.36 13CPreme G5 (Conoco- 6 2.8 Phillips)

Data for devices prepared according to General Device Process 2 are setout in Table 5.

TABLE 5 Silicon/ Graphite Graphite First Cycle Example Silicon Type typesize ratio loss vs Li (%) Example 8 Pillared particle SFG10 1.73 13.4D50 = 11.4 μm Example 9 Pillared particle SFG6 3.26 12.4 D50 = 11.4 μmExample 10 Pillared particle HPM850 2.61 12.6 D50 = 11.4 μm ComparativePillared particle 4827 6.95 15.4 Example 6 D50 = 11.4 μm Example 11Pillared particle 1:1 mix of 2.80 13.2 D50 = 11.4 μm SFG10 & 4827Example 12 Powder SFG10 0.70 14.2 D50 = 4.6 μm Example 13 Powder SFG61.31 10.4 D50 = 4.6 μm Example 14 Powder 4827 2.80 12.7 D50 = 4.6 μmExample 15 Powder HPM850 0.92 9.3 D50 = 4.6 μm Example 17 Pillaredparticle G5 1.90 12.5 D50 = 11.4 μm

Comparative example 6, with a silicon:graphite size ratio of >4.5, has ahigher first cycle loss than other cells. This cannot be attributedsolely to the high BET surface area of the graphite used in ComparativeExample 6; the first cycle loss using the same graphite is significantlylower in Example 14 in which the silicon:graphite size ratio is below4.5.

Furthermore, Example 11 contains a 1:1 mix of the largest (SFG10) andsmallest (4827) graphite material which brings the average size and BETdown. The size ratio is less than 4.5 and the first cycle loss isreduced as compared to Comparative Example 6.

Preferably first cycle loss of devices according to the invention isless than 15% measured vs Li.

Capacity retention with cycle number for some of the devices of Table 5is shown in FIG. 8. It can be seen that capacity for Example 17 islowest. With reference to Table 4, the graphite used in Example 17 hasthe lowest BET value of the materials studied.

FIGS. 9A and 9B are SEM images of graphite CPreme G5 and graphite SFG10respectively. The CPreme G5 is more rounded than SFG10.

Without wishing to be bound by any theory, it is believed that the morerounded CPreme G5 creates fewer contact points with the silicon andother components in the composite, or the points of contact coversmaller areas, than SFG10. This is borne out by lower compositeconductivity for the coating of Example 17 measured as 0.023 S/cm,compared to the coating of Example 7 with a conductivity of 0.081 S/cm.

It will be appreciated that the shape of a particle will affect its BETvalue. Preferably, the graphite BET is at least 3 m²/g. Natural orsynthetic graphite in a high aspect ratio flake form is preferred, forexample flakes having a length, height and thickness wherein both lengthand width of the particles are each independently on average at least 5times, optionally at least 10 times, the thickness of the particles.

Example 16

A device was prepared according to General Device Process 2 except thatthe silicon used had a D50 value of 11.5 microns and 4 wt % of SFG6 wasused, such that the composite anode comprised Si:Binder:Graphite:Cmix ina ratio of 70:14:4:12 in which Cmix is a mixture of VGCF:CNT:CB in theweight ratio 5:5:2

Details of graphite used in Examples 8 and 9 (reproduced from Tables 4and 5 above) and Example 16 are summarised in Table 6.

TABLE 6 Silicon/ Graphite Graphite Graphite D50 Graphite Example typeamount (wt %) (microns) size ratio Example 8 SFG10 12 6.6 1.73 Example 9SFG6 12 3.5 3.26 Example 16 SFG6 4 3.5 3.29

The cell had a capacity retention of 133 cycles to reach 80% ofcapacity. With reference to FIG. 8, this falls between the capacityretention for Examples 8 and 9, described in Table 4.

This indicates that as weight percentage of graphite rises the preferredgraphite size falls.

The invention has been described with reference to electroactive siliconas the first particulate electroactive material, however it will beunderstood that the invention may be applied to other electroactivematerials that have a bulk volume expansion of more than 10% when fullylithiated or is capable of having a specific capacity of greater than300 mAh/g, or any metal or semi-metal that can reversibly form an alloywith lithium. Other exemplary electroactive materials are tin;aluminium; electroactive compounds including oxides, nitrides,fluorides, carbides and hydrides, for example compounds of tin,aluminium and silicon; and alloys thereof.

The invention has been described with reference to rechargeable lithiumion batteries, however it will be understood that the invention may beapplied to metal ion batteries generally, and moreover that theinvention may be applied to other energy storage devices, for examplefuel cells.

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that variousmodifications, alterations and/or combinations of features disclosedherein will be apparent to those skilled in the art without departingfrom the scope of the invention as set forth in the following claims.

The invention claimed is:
 1. A composition comprising a firstparticulate electroactive material, 0.5-6 wt % of a particulate graphitematerial and a binder, wherein 50% of the total volume of each of thefirst particulate electroactive material and the particulate graphitematerial is made up of particles having a measured spherical equivalentvolume diameter equal to or less than a particle size D₅₀ and wherein aratio of D₅₀ particle size of the first particulate electroactivematerial:D₅₀ particle size of the particulate graphite material is atleast 2:1 and no more than 4.5:1, wherein the first particulateelectroactive material comprises silicon-comprising particles, andwherein the particulate graphite material comprises graphite flakeswherein (i) both a length and a width of the graphite flakes are eachindependently on average at least 5 times a thickness of the flakes,and/or (ii) the thickness of the flakes is less than 1 micron.
 2. Thecomposition according to claim 1, wherein the first particulateelectroactive material comprises silicon-comprising structured particleshaving a particle core and electroactive pillars extending from theparticle core.
 3. The composition according to claim 2, wherein theelectroactive pillars are silicon-comprising pillars.
 4. The compositionaccording to claim 3, wherein the silicon-comprising particles consistessentially of n- or p-doped silicon and wherein the pillars areintegral with the core.
 5. The composition according to claim 1, whereinthe first particulate electroactive material is provided in an amount ofat least 50 wt % of the composition.
 6. The composition according toclaim 1, wherein the composition comprises at least one elongatenanostructure material.
 7. The composition according to claim 6, whereinthe composition comprises at least two elongate nanostructure materials.8. The composition according to claim 7, wherein a first elongatenanostructure:second elongate nanostructure weight ratio is in a rangeof 2.5:1 to 20:1.
 9. The composition according to claim 7, wherein eachof the elongate nanostructure materials has an aspect ratio of at least50.
 10. The composition according to claim 7, wherein the first andsecond elongate nanostructure materials are each independently selectedfrom carbon nanotubes and carbon nanofibres.
 11. The compositionaccording to claim 6, wherein the elongate nanostructure materials arecarbon and is provided in a total amount in a range of 2-25 weight % ofthe composition.
 12. The composition according to claim 6, wherein theat least one elongate nanostructure material has a functionalisedsurface.
 13. The composition according to claim 1, wherein a crystallitelength Lc of the particulate graphite material is at least 50 nm. 14.The composition according to claim 1, wherein the composition furthercomprises carbon black.
 15. A metal-ion battery comprising an anode, acathode, and an electrolyte between the anode and the cathode whereinthe anode comprises a composition comprising: a first particulateelectroactive material; 0.5-6 wt % of a particulate graphite material;and a binder, wherein 50% of the total volume of each of the firstparticulate electroactive material and the particulate graphite materialis made up of particles having a measured spherical equivalent volumediameter equal to or less than a particle size D₅₀ and wherein a ratioof D₅₀ particle size of the first particulate electroactive material:D₅₀particle size of the particulate graphite material is at least 2:1 andno more than 4.5:1, wherein the first particulate electroactive materialcomprises silicon-comprising particles, and wherein the particulategraphite material comprises graphite flakes wherein (i) both a lengthand a width of the graphite flakes are each independently on average atleast 5 times a thickness of the flakes, and/or (ii) the thickness ofthe flakes is less than 1 micron.
 16. A method of forming a metal-ionbattery, the metal ion battery comprising an anode, a cathode, and anelectrolyte between the anode and the cathode, the method comprisingsteps of: forming an anode by depositing a slurry onto a conductivematerial, the slurry comprising a composition comprising: a firstparticulate electroactive material, 0.5-6 wt % of a particulate graphitematerial, and a binder, wherein 50% of the total volume of each of thefirst particulate electroactive material and the particulate graphitematerial is made up of particles having a measured spherical equivalentvolume diameter equal to or less than a particle size D₅₀ and wherein aratio of D₅₀ particle size of the first particulate electroactivematerial:D₅₀ particle size of the particulate graphite material is atleast 2:1 and no more than 4.5:1, wherein the first particulateelectroactive material comprises silicon-comprising particles, andwherein the particulate graphite material comprises graphite flakeswherein (i) both a length and a width of the graphite flakes are eachindependently on average at least 5 times a thickness of the flakes,and/or (ii) the thickness of the flakes is less than 1 micron; andevaporating the at least one solvent.
 17. The composition according toclaim 1, wherein the first particulate electroactive material comprisesporous silicon-comprising particles.
 18. The composition according toclaim 1, wherein the first particulate electroactive material comprisesmacroporous silicon-comprising particles.
 19. The composition accordingto claim 1, wherein the first particulate electroactive materialcomprises silicon-comprising particles with a smallest dimension of lessthan 1 μm.
 20. The composition according to claim 1, wherein the firstparticulate electroactive material comprises silicon-comprisingparticles with a largest dimension of no more than 100 μm.
 21. Thecomposition according to claim 1, wherein the particulate graphitematerial has D₅₀ particle size of less than 50 μm.
 22. The compositionaccording to claim 1, wherein the particulate graphite material isgraphene-based particles comprising a plurality of stacked graphenesheets.
 23. The composition according to claim 2, wherein theelectroactive pillars are silicon-comprising nanowires.
 24. Thecomposition according to claim 6, wherein the first elongatenanostructure has a mean average diameter of at least 100 nm.
 25. Thecomposition according to claim 1, wherein the ratio is in a range of2:1-4:1.
 26. The composition according to claim 1, wherein the ratio isin a range of 3:1-4:1.
 27. The composition according to claim 7, whereinthe second elongate carbon nanostructure material has a mean averagediameter of no more than 90 nm.
 28. The composition according to claim7, wherein the second elongate carbon nanostructure material has a meanaverage diameter in the range of 40-90 nm.
 29. The composition accordingto claim 7, wherein the first carbon elongate nanostructure material isa carbon nanofibre and the second elongate carbon nanostructure materialis a carbon nanotube.
 30. The composition according to claim 1, whereina crystallite length Lc of the particulate graphite material is at least100 nm.
 31. The composition according to claim 1, wherein theparticulate graphite material has a BET of at least 10 m²/g.
 32. Thecomposition according to claim 14, wherein the carbon black is providedin an amount of at least 0.5 weight % of the composition and less than10 wt % of the composition.
 33. The composition according to claim 14,wherein the carbon black is provided in an amount of at least 0.5 weight% of the composition and less than 4 wt % of the composition.
 34. Thecomposition according to claim 6, wherein the at least one elongatenanostructure material has a surface functionalised with anitrogen-containing group or an oxygen containing group.
 35. Thecomposition according to claim 1, wherein the first particulateelectroactive material comprises silicon-comprising structured particlesselected from the group consisting of (i) particles having a core andpillars extending from the core and (ii) particles having pores on theparticle surface or pores throughout the particle volume.
 36. Thecomposition according to claim 1, wherein the particulate graphitematerial has a BET of no more than 100 m²/g.